I have this problem too. Tried Volume Plus App, no change (but I like the app).
I put notification folder on SD card, select my own notification tone and it works but volume is very low. I put the same tone in my ringtone folder and select it as a ringtone it is way louder.
Edit:
Just tried fresh copy of the mp3 into My Music folder, then using App "Ringtone Maker" edited with volume boost and save as notification. Still no change.